Abstract : The ever increasing human population and the resultant increase in anthropogenic activities have led to a gradual decline in
groundwater quality across many regions of the world. Amongst the many contaminants, nitrates are currently considered as one of
the major causes of groundwater pollution mainly due to their high solubility in water. Frequently attributed to agricultural activities,
nitrates can also come from other sources from urban and suburban areas, particularly due to the presence of on-site sanitation
systems.
To reclaim groundwater quality by acting on the most significant and dominant sources, it is necessary to precisely determine the
origin of nitrates.
A robust approach based on the combined use of multiple environmental tracers was implemented on two watersheds characterized
by mixed pollution sources (on-site sanitation and agricultural activities). These two watersheds are located in Reunion Island (Indian
Ocean), where on-site sanitation dominates, with a significant non-compliance rate: it is estimated that in Reunion Island the on-site
sanitation is found in 60% of houses, with a non-compliance rate close to 80%.
The hydrochemical analysis coupled with the spatial analysis of land use/land cover helped us to characterize the different
groundwater facies. Water isotope ratios (delta 18O and delta 2H) were used to determine the average recharge altitude, while the
rare earths, pharmaceutical compounds, nitrate isotopic ratios (delta 15N and delta 18O), boron isotopes 10B and 11B allowed us
discriminate between the agricultural and urban sources of nitrates in groundwater. The relative contribution of pollution from on-site
sanitation and agricultural activities was determined.
Finally, CFC's and SF6 were used to calculate the apparent ages of water and identify the temporal effectiveness of measures
implemented in the watershed, characterized by mixed pollution sources.